Lock (water transport)
From Wikipedia, the free encyclopedia
On navigable waterways, a lock is a particular type of device for raising or lowering boats between stretches of water at different levels. The distinguishing feature of a lock is a fixed chamber whose water level can be varied; whereas in a boat lift or canal inclined plane, it is the chamber itself which moves.
Locks are used to make a river more easily navigable, or to allow a canal to cross country that is not level.
The term airlock was coined for a similar device used to allow persons to pass to and from a location in which a particular atmosphere is maintained, such as underwater, in space, or in a clean room.
Contents |
[edit] Use of locks in river navigations
A lock is required when a stretch of river is made navigable by bypassing an obstruction such as a rapid, dam, or mill weir — because of the change in river level across the obstacle.
In large scale river navigation improvements, weirs and locks are used together. A weir will increase the depth of a shallow stretch, and the required lock will either be built in a gap in the weir, or at the downstream end of an artificial cut which bypasses the weir and perhaps a shallow stretch of river below it. A river improved by these means is often called a Waterway or River Navigation (see example Calder and Hebble Navigation).
The lowest lock on a navigable river separates the tidal and non-tidal stretches. Sometimes a river is made entirely non-tidal by constructing a Sea Lock directly into the estuary.
In more advanced river navigations, more locks are required.
- Where a longer cut bypasses a circuitous stretch of river, the upstream end of the cut will often be protected by a flood lock.
- The longer the cut, the greater the difference in river level between start and end of the cut, so that a very long cut will need additional locks along its length. At this point, the cut is, in effect, a canal.
[edit] Use of locks in canals
Early completely artificial canals, across fairly flat countryside, would get round a small hill or depression by simply detouring (contouring) around it. As engineers became more ambitious in the types of country they felt they could overcome, locks became essential to effect the necessary changes in water level without detours that would be completely uneconomic both in building costs and journey time. Later still, as construction techniques improved, engineers became more willing to barge directly through and across obstacles by constructing long tunnels, cuttings, aqueducts or embankments, or to construct even more technical devices such as inclined planes or boat lifts. However, locks continued to be built to supplement these solutions, and are an essential part of even the most modern navigable waterways.
[edit] Basic construction and operation
All locks have three elements:
- A watertight chamber connecting the upper and lower canals, and large enough to enclose one or more boats. The position of the chamber is fixed, but its water level can vary.
- A gate (often a pair of "pointing" half-gates) at either end of the chamber. A gate is opened to allow a boat to enter or leave the chamber; when closed, the gate is watertight.
- A set of lock gear to empty or fill the chamber as required. This is usually a simple valve (traditionally, a flat panel lifted by manually winding a rack and pinion mechanism) which allows water to drain into or out of the chamber; larger locks may use pumps.
The principle of operating a lock is simple. For instance, if a boat travelling downstream finds the lock already full of water:
- The entrance gates are opened and the boat sails in.
- The entrance gates are closed.
- A valve is opened, this lowers the boat by draining water from the chamber.
- The exit gates are opened and the boat sails out.
Notes
- If the lock was empty, the boat would have had to wait 5-10 minutes while the lock was filled.
- For a boat travelling upstream, the process is reversed: for instance, the chamber is filled by opening a different valve which allows water to enter the chamber from the upper level.
- The whole operation will usually take between 10 and 20 minutes, depending on the size of the lock, and whether it was originally set "for" the boat.
- Boaters approaching a lock are usually pleased to meet another boat coming towards them, because this boat will have just exited the lock on their level and therefore set the lock in their favour — saving some work and some 5-10 minutes. (This is not true for staircase locks, where it is quicker for boats to go through in convoy.)
[edit] Details and terminology
For simplicity, this section describes a basic type of lock, with a pair of gates at each end of the chamber and simple rack and pinion paddles raised manually by means of a detachable windlass operated by the boat's shore crew. This type can be found all over the world, but the terminology here is that used on the British canals. A subsequent section explains common variations.
[edit] Rise
The change in water-level effected by the lock. The deepest lock on the English canals is Tuel Lane Lock on the Rochdale Canal with rise of about 20 feet. A more typical (English) rise would be 8-12 feet (though even shallower ones can be encountered).
[edit] Pound
The level stretch of water between two locks (on a river, the corresponding term is commonly reach). The lock allows a boat to move between the pound above it (upper pound) and the pound below it (lower pound).
[edit] Chamber
The main feature of a lock. It is a watertight (masonry, brick, or concrete) enclosure which can be sealed off from the pounds at either end by means of gates. The chamber may be the same size (plus a little manouevering room) as the largest vessel for which the waterway was designed; sometimes larger, to allow more than one such vessel at a time to use the lock. The chamber is said to be "full" when the water level is the same as in the upper pound; and "empty" when the level is the same as in the lower pound. (If the lock has no water in it at all, perhaps for maintenance work, it might also be said to be empty, but a less-confusing term for this is "drained".)
[edit] Cill
A narrow horizontal ledge protruding a short way into the chamber from below the upper gates. Allowing the rear of the boat to "hang" on the cill is the main danger one is warned to guard against when descending a lock, and the position of the forward edge of the cill is usually marked on the lockside by a white line. The edge of the cill is usually curved, protruding less in the centre than at the edges.
[edit] Gates
The watertight doors which seal off the chamber from the upper and lower pounds. Each end of the chamber is equipped with a pair of swinging oak, elm (or now sometimes steel) half-gates. When closed they meet at an angle like a chevron pointing upstream (this arrangement is often called pointing doors) and a very small difference in water-level squeezes the closed gates securely together. This reduces any leaks from between them and prevents their being opened until water levels have equalised. If the chamber is not completely full, the top gate is secure; and if the chamber is not completely empty, the bottom gate is secure (in normal operation, therefore, the chamber cannot be open at both ends). A lower gate is taller than an upper gate, because the upper gate only has to be tall enough to close off the upper pound, while the lower gate has to be able to seal off a full chamber. The upper gate is as tall as the canal is deep, plus a little more for the balance beam, winding mechanism, etc; the lower gate's height equals the upper gate plus the lock's rise.
[edit] Balance beam
A long arm projecting from the landward side of the gate over the towpath. As well as providing leverage to open and close the heavy gate, the beam also balances the weight of the gate in it socket, and so allows the gate to swing more freely.
[edit] Paddle
The simple valves by which the lock chamber is filled or emptied. A paddle is simply a sliding wooden panel which when "lifted" (slid up) out of the way allows water to either enter the chamber from the upper pound or flow out to the lower pound. A gate paddle simply covers a hole in the lower part of a gate; a more sophisticated ground paddle blocks an underground culvert. There can be up to 8 paddles (two gate paddles and two ground paddles at both upper and lower ends of the chamber) but there will often be fewer.
[edit] Windlass ("lock key")
A detachable crank used for opening lock paddles (NOT the winding mechanism itself). The simplest windlass is an iron rod (circular section, about half an inch in diameter and three feet long) bent into an L-shape, with a square socket at one end for fitting onto the "stub" protruding from the lock winding gear. Most have two sizes of socket, for different locks standards. There may be refinements such as an extensible handle (for stiff paddles), or a free-rotating cylindrical sleeve around the handle (to prevent blisters). On canals with well-maintained (easy-to-lift) paddle gear, crews often prefer to carry a smaller, lighter windlass made of aluminium. Most boats carry three or four windlasses of different types.
[edit] Winding gear / paddle gear
The mechanism which allows paddles to be lifted (opened) or lowered (closed). Typically, a square-section stub emerges from the housing of the winding gear. This is the axle of a sprocket ("pinion") which engages with a toothed bar ("rack") protruding from the top of the paddle. A member of the boat's shore crew engages the square hole of their windlass onto the end of the axle and turns the windlass perhaps a dozen times. This rotates the pinion and lifts the paddle. A pawl engages with the rack to prevent the paddle from dropping inadvertently while being raised, and to keep it raised when the windlass is removed, so that the operator can attend to other paddles (it is considered discourteous and wasteful of water to leave a paddle open after a boat has left the lock). Before the paddle can be lowered, the pawl has to be disengaged.
[edit] "Turning" a lock
This can simply mean emptying a full lock or filling an empty one (We entered the lock, and it only took us five minutes to turn it). It is used more often to refer to a lock being filled or emptied while you are not in it (The lock was turned for us by a boat coming the other way) and particularly when there is no boat in it at all (The lock was set for us, but the crew of the boat coming the other way turned it before we got there).
[edit] Variations
Not all locks work exactly as described above, and the terminology changes, too ...
- Single gates on narrow canals (locks approx. 7 feet / 2.1 m wide)
- A few narrow locks imitate wide locks in having paired gates at both ends (eg Bosley, on the Macclesfield canal)
- On most English narrow canals however, the upper end of the chamber is closed by a single gate the full width of the lock. This was cheaper to construct and is quicker to operate, as only one gate needs to be opened.
- Some narrow locks (eg on Birmingham Canal Navigations) go even further. They have single gates at the lower end also. This speeds up passage, even though single lower gates are very heavy (heavier than a single upper gate, because the lower gate is taller) and the lock has to be longer (a lower gate opens INTO the lock, it has to pass the bow or stern of an enclosed boat, and a single gate has a wider arc than two half-gates).
- Alternate forms of gates
- Steel Gates. Some locks (particularly modern commercial ones) use gates made of steel, but even very large locks still use essentially the same swinging gate design, with the exception of some low-head locks that use sliding gates (see Kiel Canal).
- Guillotine Gates. Some locks have vertically moving steel gates — these are quite common on river navigations in East Anglia. Sometimes just one of the pairs of swinging gates is replaced by a guillotine: for instance at Salterhebble Locks, where space is restricted by a bridge.
- Alternate paddle gear
- Some manually-operated paddles do not require a detachable handle (windlass) because they have their handles ready-attached.
- On the Leeds and Liverpool Canal, paddles are raised by turning what is in effect a large horizontal wingnut (butterfly nut) lifting a screw-threaded bar attached to the top of the paddle.
- On the Calder and Hebble Navigation, some paddle gear is operated by repeatedly inserting a Calder and Hebble Handspike (length of 4" by 2" hardwood) into a ground-level slotted wheel and pushing down on the handspike to rotate the wheel on its horizontal axis.
- Lock Keepers. Some locks are operated (or at least supervised) by professional lock keepers. This is particularly true on commercial waterways, or where locks are large or have complicated features that the average leisure boater may not be able to operate successfully. For instance, although the upper Thames (England) is almost entirely a leisure waterway, the locks are usually staffed.
- Powered operation. On large modern canals, especially VERY large ones such as ship canals, the gates and paddles are too large to be hand operated, and are operated by hydraulic or electrical equipment. Even on smaller canals, some gates and paddles are electrically operated, particularly if the lock is regularly staffed by professional lockkeepers. Powered locks are usually still filled by gravity, though some very large locks use pumps to speed things up.
- Fish Ladders. The construction of locks on rivers obstructs the passage of fish such as trout going upstream to spawn. Measures such as a fish ladder are often taken to counteract this.
[edit] Illustrations
Pictures below depict various lock operations:
 Tug and barge in lock when full. |
 Lock emptied for maintenance. |
 Lock emptied for maintenance. |
 Lock emptied for maintenance. |
 Gate opening mechanism. |
In November 2004 the Hiram M. Chittenden Locks was emptied for maintenance. This provided an opportunity to visualize how a lock works without water obscuring the bottom of the lock. For reference the picture above on the far left shows the lock in operation with a tug and barge, loaded with sand and gravel bound for a nearby concrete mixing plant, waiting for the gates to open. The cutout in the side wall in the bottom left corner of the picture contains the gate when open.
The lock has three pairs of gates, one pair at each end and one pair in the middle so that half the length of the lock can be used when whole length of the lock is not required thus saving water. The last three pictures show from left to right, the low water end of the lock, the center pair of gates and the high water end of the lock. The person walking on the bottom near the middle of the lock in the second picture from the left gives a measure of the size of the lock. In the pictures of both ends of the lock the string of penstock openings are visible along the sides at the bottom. The water entering and leaving the lock flows by gravity through these openings. It requires around 15 minutes to fill or empty the lock.
[edit] Special cases
[edit] Lock flights
Loosely, a flight of locks is simply a series of locks in close-enough proximity to be identified as a separate group. For many reasons, a flight of locks is preferable to the same number of locks spread more widely: crews are put ashore and picked up once, rather than multiple times; transition involves a concentrated burst of effort, rather than a continually-interrupted journey; a lockkeeper may be stationed to help crews through the flight quickly; and where water is in short supply, a single pump can recycle water to the top of the whole flight. The need for a flight may be purely determined by the lie of the land, but it is possible to purposely group locks into flights by using cuttings or embankments to "postpone" the height change. Examples: Caen Hill Locks, Devizes.
A lock flight should not be confused with a lock staircase. In a flight, each lock has its own upper and lower gates, there is a pound (however short) between each pair of locks, and the locks are operated in the conventional way.
[edit] Staircase locks
When a very steep gradient has to be climbed, a lock staircase is used. A staircase can be thought of as a "compressed" flight, where the intermediate pounds have disappeared, and the upper gate of one lock is also the lower gate of the one above it. However, it is incorrect to use the terms staircase and flight interchangeably: because of the "loss" of the intermediate pounds, operating a staircase is very different from operating a flight. It can be more useful to think of a staircase as a single lock with intermediate levels (the top gate is a normal top gate, and the intermediate gates are all as tall as the bottom gate).
Examples of famous large staircases in England include Foxton and Bingley. Two-rise staircases are more common: Snakeholme Lock and Struncheon Hill Lock on the Driffield Navigation were converted to staircase locks after low water levels hindered navigation over the bottom cill at all but the higher tides — the new bottom chamber rises just far enough to get the boat over the original lock cill. In China, the recently completed Three Gorges Dam includes a double five-step staircase for large ships, and a ship lift for vessels of less than three thousand metric tons.
Many inexperienced boaters find operating a staircase the stuff of nightmares. The key worries (apart from simply being paralysed with indecision) are either sending down more water than the lower chambers can cope with (flooding the towpath, or sending a tidal wave along the canal) or completely emptying an intermediate chamber (although this shows that a staircase lock can be used as an emergency dry dock). To avoid these mishaps, it is usual to have the whole staircase empty before starting to descend, or full before starting to ascend.
One striking difference in using a staircase (compared with a single lock, or a flight) is the best sequence for letting boats through. In a single lock (or a flight with room for boats to pass) it is obvious that boats should ideally alternate in direction. In a staircase, however, it is quicker for a boat to follow a previous one going in the same direction. Because of this, some staircases have "up" and "down" time-slots.
As with a flight, it is possible for more than one boat to be in a staircase at the same time, but managing this without waste of water requires expertise. On English canals, a staircase of more than two chambers is usually staffed: the locky at Bingley (looking after both the "5-rise" and the "3-rise") has worked there for more than 20 years and ensures that there are no untoward events and that boats are moved through as speedily and efficiently as possible. Such expertise permits miracles of boat balletics: it is possible for boats travelling in opposite directions to pass each other halfway up the staircase; or at peak times, to have all the chambers full simultaneously with boats travelling in the same direction.
[edit] Doubled / Paired / Twinned Locks
Locks can be built in parallel (ie side by side). This can be called Doubling, Pairing, or Twinning. There are several examples (in this case called "double locks") on one stretch of the Trent and Mersey Canal). Twinning gives advantages in speed : avoiding holdups at busy times; or increasing the chance of a boat finding a lock set in its favour. Also, there can be water savings: the locks may be of different sizes, so that a small boat does not need to empty a large lock; or each lock may be able to act as a side pond for the other.
These terms can also (in different places or to different people) mean either a two-chamber staircase, or just a flight of two locks (as at Thornhill Double Locks on the Calder and Hebble Navigation). Also, "double lock" (less often, "twin lock") is often used by novices on the English canals to mean a wide (14ft) lock, presumably because they are "double" the width of a narrow lock, and allow two narrow boats to "double up".
[edit] Stop locks
A "stop" lock is a (very) low-rise lock built at the junction of two (rival) canals to prevent water from passing between them. During the competitive years of the English waterways system, an established canal company would often refuse to allow a connection from a newer, adjacent one. This situation created the Worcester Bar in Birmingham, where goods had to be transhipped between boats on rival canals only feet apart.
Where a junction was built, either because the older canal company saw an advantage in a connection, or where the new company managed to insert a mandatory connection into its Act of Parliament, then the old company would seek to protect (and even enhance) its water supply. They would insist that, at the junction, the newer canal must be at a higher level. Even though the rise might only a few inches, the difference in levels still required a lock — called a stop lock, because it was to stop water flowing continuously between the newer canal and the older, lower one. The lock would be under the control of the new company, and the gates would, of course, "point" uphill towards the newer canal, protecting its water supply (but "losing" a lockful everytime a boat went through).
When variable conditions meant that a higher water level in the new canal could not be guaranteed, then the older company would also build a stop lock (under its own control, with gates pointing towards its own canal) which could be closed when the new canal was low. This resulted in a sequential pair of locks, with gates pointing in opposite directions: one example was at Hall Green near Kidsgrove, where the southern terminus of the Macclesfield Canal joined the "Hall Green branch" of the earlier Trent And Mersey Canal.
Many stop locks were removed or converted to a single gate after nationalisation in 1948. Hall Green Stop Lock remains, but as a single lock: the extra lock was removed because the lowering of the T&M's summit pound (to improve Harecastle Tunnel's "air draught" — its free height above the water level) meant that the T&M would always be lower than the Macclesfield. The Hall Green Branch is now considered to be an extension of the "Macc", which now meets the T&M at Hardings Wood junction (just short of the Harecastle Tunnel north portal).
[edit] Flood locks
A flood lock is to prevent a river from flooding a connected waterway. It is typically installed where a canal leaves a river. At normal river levels, the lock gates are left open, and the height of the canal is allowed to rise and fall with the height of the river.
However, if the river floods beyond a safe limit for the canal, then the gates are closed (and an extra lock created) until the river drops again. Because this is a true lock, it is possible for boats to leave the canal for the flooded river despite the difference in water levels (though this is not likely to be wise) or (more sensibly) to allow boats caught out on the flood to gain refuge in the canal.
Note that if the canal is simply a navigation cut connecting two stretches of the same river, the flood lock will be at the upstream end of the cut (the downstream end will have a conventional lock).
Flood locks which have been used only as flood gates (see below) are often incapable of reverting to their former purpose without refurbishment. That is, where only outer gates are ever closed (probably because a waterway is not a true commercial one, and therefore there is no financial imperative for a boat to venture out onto a flooded river) inner gates soon suffer from lack of maintenance. A good example is on the Calder and Hebble Navigation, where structures referred to in the boating guides as "Flood Locks" are clearly only capable of being used for flood-prevention, not for "penning" boats to or from the river in flood.
[edit] Flood gates
A flood gate is a poor man's flood lock. Only one set of gates exist, and so when the river is higher than the canal, the gates are closed and navigation ceases. These are quite common in the French inland waterways system. A flood gate is also sometimes called a stop lock. Flood gates may also be used to separate long canal pounds or protect, in case of dam rupture, the surrounding area if this is lower than the water level of the canal.
[edit] Bi-directional gates and locks
Where a lock is tidal (i.e. one side of the lock has water whose level varies with the tide) or where a canal meets a river whose level may vary, the water on the tidal or river side (the "downstream" side) may rise above the water on the normal "upper" side. The "upstream" pointing doors will then fail to do their job, and will simply drift open. To prevent water flowing the wrong way through the lock, there will need to be at least one set of gates pointing in the "wrong" direction. If it is desirable that boats can use the lock in these circumstances, then there needs to be a full set of gates pointing towards the tidal or river side. The usual method is to have gates pointing in opposite directions at both ends of the chamber (alternatively, the "paired stop lock" arrangement of two separate sequential locks pointing in opposite directions would work here — but would require an extra chamber). If navigation is not required (or impossible) at one "extreme" (e.g. allow navigation above mid-tide, but just prevent the canal emptying at low tide) then it is only necessary to have one set of bi-directional gates.
[edit] Sea Locks
A lock connecting a canal or river directly with the estuary (or beach). All sea-locks are tidal.
[edit] Tidal locks
Loosely, any lock connecting tidal with non-tidal water. This includes a lock between a tidal river and the non-tidal reaches; or between a tidal river and a canal; or a sea lock. However, the term usually refers specifically to a lock whose method of operation is affected by the state of the tide. Examples:
- A canal joining a river whose levels are always lower than the canal. All that is needed is an ordinary lock, with the gates pointing up the canal. The lock is used normally so long as the tide is high enough to float boats through the lower gates. If near low tide the lock becomes unusable, then the gates can be barred (and simply become a "reverse flood gate", holding water in the canal). This arrangement also applies to some sea locks (eg Bude Canal).
- A canal joining a river which is normally below it, but which can rise above it (at very high tides, or after heavy rain). One pair of gates can be made bidirectional, ie the inward-pointing gates would be supplemented by a pair pointing out to the river. When the river is higher than the canal, the normal gates would just drift open, but the additional pair of gates can be closed to protect the canal, and prevent navigation to the river. In effect, we have simply added a flood gate.
- As above, but where it is safe to navigate even when the river is higher than the canal. The lock will be fully bidirectional (two pairs of oppositely pointing gates at each end) to allow boats to pass at any normal river levels. At extreme low or high tides unsuitable for navigation, the appropriate sets of gates are barred to prevent passage.
[edit] Very large locks
The world's largest canal lock is the "Berendrecht lock" and can be found in Antwerp, Belgium. The lock is 500 metres (1,640 feet) long, and 68 metres (223 feet) wide, and has four sliding lock gates. The size of locks cannot be compared without considering the difference in water level that they are designed to operate under. The total volume of water to be considered in any lock equals the product of its length, breadth and the difference in water levels. Lock staircases are used in an attempt to reduce the total volume of water required in relation to the amount of useful work done. The useful work done relates to the weight of the vessel and the height it is lifted. When a vessel is lowered the consumption of potential energy of the water consumed is considered. Alternatives to locks such as the Anderton Boat Lift do not rely on the consumption of water as the primary power source, are powered by motors and are designed to consume a minimum amount of water.
The 29 locks on the Mississippi River are typically 600 foot (180 m) long while tug and barge combinations are as much as 1200 feet (360 m) long consisting of as many as 15 barges and one tug. In these cases, some of the barges are locked through, using partially opened lock valves to create a current to pull the un-powered barges out of the lock where they are tied up to wait the rest of the barges and the tug to pass through the lock. It can take as much as an hour and a half to pass the lock.
[edit] History and development
[edit] Dams and weirs
In ancient times river transport was common, but rivers were often too shallow to carry anything but the smallest boats. Ancient people discovered that rivers could be made to carry larger boats by making dams to raise the water level. The water behind the dam deepened until it spilled over the top creating a weir. The water was then deep enough to carry larger boats. This dam building was repeated along the river, until there were "steps" of deep water.
[edit] Flash locks
This however created the problem of how to get the boats between these "steps" of water. An early and crude way of doing this was by means of a flash lock. A flash lock consisted essentially of a small opening in the dam, which could be quickly opened and closed. On the Thames in England, this was closed with vertical posts (known as rimers) against which boards were placed to block the gap.
When the gap was opened, a torrent of water would spill out, cattying a "downstream" boar with it, or allowing an "upstream" boat to be manhauled thorough against the flow. When the boat was through, the opening would be quickly closed again. The "gate" could also be opened to release a 'flash' downstream to enable grounded boats to get off shoals, hence the name.
This system was used extensively in Ancient China and in many other parts of the world. But this method was dangerous, many boats were sunk by the torrent of water.
[edit] Staunch
A more sophisticated device was the staunch or water gate, consisting of a gate (or pair of mitred gates) which could be closed (and held shut by water pressure) when the river was low, in order to float vessels over upstream shallows at times of low water. However, the whole upstream head of water had to be drained (by some auxilliary method approaching modern sluices) before the a boat could pass. Accordingly they were not used where the obstacle to be passed was a mill weir.
[edit] Pound lock
The natural extension of the Staunch was to provide an upper gate (or pair of gates) to form an intermediate "pound" which was all that need be emptied when a boat passed through. This is a pound lock, the type of lock seen today: known inMedieval Europe, but first used on the Grand Canal in China long before that...
The Songshi or History of the Song Dynasty, volume 307, biography 66, records how Qiao Weiyue, a high-ranking tax administrator, was frustrated at the frequent losses incurred when his grain barges were wrecked on the West River near Huai’an in Jiangsu. The soldiers at one double slipway, he discovered, had plotted with bandits to wreck heavy imperial barges so that they could steal the spilled grain. In 984 Qiao installed a pair of sluice-gates two hundred and fifty feet apart, the entire structure roofed over like a building. By siting two staunch gates so close by one another, Qiao had created a short stretch of canal, effectively a pound-lock, filled from the canal above by raising individual wooden baulks in the top gate and emptied into the canal below by lowering baulks in the top gate and raising ones in the lower.
[edit] Use of water
The main problem caused by locks is that, each time a lock goes through one fill-empty cycle, a lockful of water (tens or hundreds of thousands of gallons) is released to the lower pound. In over-simplistic terms: on a canal where only one boat will fit into a lock, a boat travelling from the summit pound to the lowest pound is accompanied on its journey by one 'personal' lockful of water. To prevent the canal from running dry, some method must be used to ensure that the water supply at the canal summit is constantly replenished at the rate that the water is being drained downwards. This is, of course much more of a problem on an artificial canal crossing a watershed than on a river navigation.
[edit] Design
When planning a canal, the designer will attempt to build a summit level with a large reservoir, or one supplied by an artificial watercourse from a distant source, or one as long as possible (to act as its own reservoir) or which cuts across as many springs or rivers as possible (or all of these).
[edit] Pumping
Where it is clear that natural supply will not be sufficient to replenish the summit level at the rate that water will be used (or to allow for unexpected periods of drought) the designer may plan for water to be back-pumped back up to the summit from lower down. Such remedies may of course be installed later, when poor planning becomes apparent, or when there is an unforeseeable increase in traffic or dearth of rain. On a smaller scale, some local pumping may be required at particular points (water is continually recycled through some locks on the Kennet and Avon canal).
[edit] Side ponds
A way of reducing the water used by a lock is to give it a reservoir whose level is intermediate between the upper and lower pounds. This reservoir can store the water drained from the upper 1/3rd of the lock as a boat descends, and release it to fill the lower 1/3rd next time a boat ascends. This saves the total amount of water lost downhill in each fill-empty cycle. On English canals, these reservoirs are called side ponds, and the gear controlling them is sometimes coloured red. This has given rise to the famous mnemonic "Red before white, you're alright; white before red, you're dead" (referring to the danger of incurring the wrath of the locky, rather than any inherent physical risk in the mechanisms themselves). On some flights of locks with short intermediate pounds, the pounds are extended sideways — in effect to provide a reservoir to ensure that the pound does not run dry (in case, for instance, the lock below leaks more than the lock above). These extended intermediate pounds are sometimes confused with side ponds.
[edit] Alternatives
As well as the 'static' approaches mentioned earlier (various types of contouring, excavating, and spanning), there were many ingenious dynamic solutions, mostly variations on the boat lift or the inclined plane. These tend to be more expensive to install and operate, but offer faster transit and waste less water. Here are four working examples...
[edit] Anderton
The Victorian Anderton Boat Lift, the world's first vertical boat lift, linking the Trent and Mersey Canal and the River Weaver in Cheshire has recently been restored.
[edit] Falkirk
The Falkirk Wheel, the world's first rotating boat lift, acts as the centrepiece of the restoration of the Forth and Clyde and Union Canals. The spectacular "Wheel" presents the 21st century's solution to replacing a flight of locks which formerly connected the canals and which were filled in 1930. The Falkirk Wheel was the winning design in a competition to design a new lock. Visitors can now take a boat trip on the Wheel and be lifted over 100 feet in a few minutes compared to the time it took when the original lock staircase operated.
[edit] Three Gorges Dam
Different solutions may be used in combination. At the Three Gorges Dam on the Yangtze River (Chang Jiang) in China there are two stair-steps of five large ship locks. In addition to this there is a ship lift (a large elevator) capable of moving a three thousand ton ship vertically in one motion.
[edit] The Caisson Lock
Around 1800 the use of Caisson locks was proposed by Robert Weldon for the Somerset Coal Canal in England. An amazing underwater boat lift, rather than a lock in the usual sense, each lock was 80 ft long and 60 ft deep and contained a closed wooden box which could take a barge. This box moved up and down in the 60ft (18.2m) deep pool of water, which never left the lock. At least one of these was built and demonstrated to the Prince Regent (later George IV), but had various engineering problems and was never successful comercially or built elsewhere.[1][2]
[edit] Ship sizes
Locks restrict the maximum size of ship able to pass through, and this size is sometimes given a simple name:
[edit] References
- ^ The Somerset Coal Canal. Bath Royal Literary and Scientific Institution. Retrieved on 2006-10-06.
- ^ History of the Caisson Lock On the Somersetshire Coal Canal. The Somersetshire Coal Canal (Society). Retrieved on 2006-10-06.
[edit] See also
- Boat lift
- Canal inclined plane
- Canal pound
- Canals of the United Kingdom
- Erie Canal — New York State
- Flash lock
- Gongoozler
- Grand Union Canal
- Kiel Canal This low head single lock uses sliding gates.
- Murray River — Australia
- Panama Canal
- Peterborough Lift Lock — Canada (Ontario), an example of boat lift
- Pound lock
- Rideau Canal — Canada (Ontario)
- Saint Lawrence Seaway — Canada
- Sault Ste. Marie Canal — Sault Ste. Marie, Ontario
- Soo Locks — Sault Ste. Marie, Michigan/Ontario
- Trent-Severn Waterway — Canada (Ontario)
- Trollhättan Municipality - lock system from lake to North Sea in Sweden
- Welland Canal — Canada (Ontario)
[edit] External links
 |
Wikimedia Commons has media related to: |
- Lock and dam on the Mississippi
- Deltaworks.Org — Philipsdam Locks, separates sweet and salt water while giving free passage to ships
- The River Wey and Wey Navigations Community Site — a non-commercial site of over 200,000 words all about the Wey Navigations and includes information and images related to canals, narrowboats and lock operation
- Sault Ste. Marie Canal National Historic Site
- Locks on the River Thames
- Interactive Lock for the original River and Rowing Museum website
|
|
Wikimedia Commons has media related to: |
